To effectively diagnose and treat chronic fatigue it is imperative to develop valid and reliable methods to measure the presence and intensity of fatigue and its effect on function, and to identify the patient characteristics associated with it. The purpose of this study is to correlate the physiological and psychosocial components of fatigue and individuals' perceptions of fatigue with performance. We will also validate a battery of self-administered assessment tools, including an exercise evaluation, that can be used (together) to characterize fatigue, monitor changes, and facilitate selection of appropriate treatments. This descriptive pilot study uses a convenience sample of a maximum of 36 adults with diagnoses of chronic fatigue syndrome, polymyositis, or rheumatoid arthritis, and a complaint of fatigue for at least 6 months. Nine evaluations are administered including physiological, psychosocial and performance measures. We have evaluated 29 patients to date.